Your post is somewhat cryptic.
When you sayWhen I try to open gnome from CLI using "gnome-session"
did you typed gnome-session on the CLI
or did you echo "exec gnome-session" > .xinitrc
then
startx
?????

Also, IIRC, while GNOME works perfectly well on OpenBSD, the GNOME implementation was said to have been updated (undeadly.org) for XFCE4 to work.

As the OpenBSD implementation of GNOME does not follows the pure GNOME installation with *all* possibly (even rarely) needed dependencies a la FreeBSD, I doubt pkg_add 'ing only *desktop and *session would give you a full working environment.
AFAIK, on OpenBSD, there is no way to install a "Gnome-light" environment with a meta-package.

If you want GNOME, I suggest you install it via the "source", that is
cd /usr/ports/x11/gnome && make install
you can set your mk.conf to preferably use packages when available and end up installing from packages only.
The ports/packages infrastructure of OpenBSD is second to none for installing and upgrading applications. Need to read some man pages.

Beginning with 4.3, gnome-session comes with a README.OpenBSD which I have replicated here for you:

Quote:

The OpenBSD/GNOME desktop

* Introduction
The gnome-session package will install a minimal working GNOME desktop.
It provides you with the basic interface (including themes), a control
panel, a terminal, a text editor and a lot more other utilities.

This file will briefly explain to you how to make this experience more
pleasant.

* Installing more packages
If your needs aren't satisfied yet, there are many other packages
you can install. The most important ones will be mentioned by category:

* Startup and more documentation
One can log in using the login manager, GDM. Or by using the 'startx' command.
In order to use the latter, add a line like the following to your .xinitrc
or .xsession script: 'exec /usr/local/bin/gnome-session' .

Yes you have to. If all your users run graphical user interfaces and this computer is more a desktop than a server, you may also try GDM - this will allow you to boot directly into the graphical user interface and log in from that interface.

__________________
"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius -- and a lot of courage -- to move in the opposite direction."

If using XDM or GDM, each user needs a .xsession file in its home directory. The .xsession file is used in the same way as the .xinitrc file; in fact they may be the same file, through the use of the ln(1) command.

The "root" user's home directory is /root. A normal user's home directory typically is created in a subdirectory of /home, but the admin may put it anywhere.

Shredder, it seems to me that you need to learn to use a Unix text editor; the most commonly used editors are vi or emacs. The vi(1) program is built in, and there is a subset of the emacs editor also, called mg(1). There are zealots in both camps. I happen to have learned vi many decades ago, so that is what I use. But it matters not what you pick, so long as you learn one of them.

If you'd like to see what a working OpenBSD system with Gnome already installed looks like, I have a LiveDVD of OpenBSD 4.3 with all of Gnome available for download. You can examine .xinitrc and .xsession files (they are the same file on this system), use pkg_info(1) to see what's installed, and see if you like the Epiphany browser or other Gnome tools. If you start the LiveDVD in "graphical" mode it will start XDM for you, if you start in "console" mode you can issue the startx command yourself.

And if I have multiple users should I make .xinitrc in each of their home folders?

This depends upon whether users need/want customization or whether the defaults are sufficient.

When startx is invoked, the script checks for the presence of ~/.xinitrc (note the period...). If present, this file is executed. If not, /etc/X11/xinit/xinitrc is executed (note no period...). In this manner users can customize how X starts by changing the contents of ~/.xinitrc. Note that the default window manager is fvwm(1) as can be found at the bottom of /etc/X11/xinit/xinitrc.

If you want to change the defaults (window manager, applications started at X's invocation, etc.), edit /etc/X11/xinit/xinitrc.